The present invention generally relates to a multi-cavity injection molding apparatus for optical disc substrates such as compact discs, and more particularly to a multi-cavity injection molding apparatus which uses a plurality of single cavity molds.
The molding of optical discs, such as CDs, CD RWs, DVDs and miniature optical formats, is generally known. Typically, an optical disc substrate is molded from a polymer resin using an injection molding apparatus. Data is stored on the optical disc by the creation of dark and bright spots (such as by the creation of pits, the use of photosensitive dyes, or the use of phase change media). The dark and bright spots may be formed during the manufacturing process or later during an optical write process, such as in a CD RW disc.
Because recorded information is read by utilizing optical characteristics of the disc, it is important that the disc is uniform in both optical characteristics and physical dimensions. Variations in the disc may induce errors in the writing of information to the disc, or in the reading of information from the disc. Variations may be caused by non-uniform resin properties, variations in dimensions of the disc, warping of the disc, etc.
The ability to minimize or eliminate such variations of the disc becomes increasingly important as the volume of information stored on the discs increases, and as the speed of the discs increases. For example, the track density of a DVD may be in excess of two to three times the track density of an audio CD. Thus, a flaw in the disc which may not affect the reading or writing of information to the disc at low densities or slow disc speeds may cause significant problems at higher track densities and disc speeds. Accordingly, discs which store information at high densities and are used in high speed disc drives are more susceptible to errors in the disc optical properties or dimensions, and it is increasingly important to maintain the optical disc within acceptable tolerances.
Because of the tight tolerances required for high density optical discs, such discs are typically made in an injection molding process which utilizes a single cavity mold. Single cavity molds are used because they provide a more uniform and consistent flow of resin into the mold cavity and more uniform thermal characteristics for the cooling of the molded disc. Non-uniform thermal characteristics of the mold (such as uneven temperatures across the mold, differing heat transfer rates, etc.) may lead to non-uniform curing of the resin, which in turn may alter the optical characteristics of the disc or cause warping of the disc as it cools. As noted above, such variations in the disc are unacceptable for high density information storage.
Although single cavity molds typically provide the best uniformity in the optical discs, they have the deficiency of rather inefficient productivity. That is, only a single optical disc may be formed at one time. The molding of each disc is necessarily separated by the steps of opening the mold, ejecting the molded object, closing the mold, and repeating the process. Clearly, multicavity molding would provide greater efficiency because multiple discs could be molded in a single step. The multicavity molding of optical discs is known, such as described in U.S. Pat. No. 5,648,105.
Although multicavity molding of optical discs has the advantage of more efficient production, it also has several disadvantages. In particular, multicavity molds suffer the disadvantage of non-uniform thermal characteristics surrounding the individual cavities of the multicavity mold. Thus, cavities (or portions of cavities) of the multicavity mold which are adjacent the edge of the mold may stay cooler or experience faster cooling than those cavities (or portions of cavities) which are closer to the center of the multicavity mold. As noted above, non-uniform thermal characteristics of the mold lead to variations in the optical and dimensional properties of the discs which render them unsuitable for use in high density storage applications.
Multicavity molds have other disadvantages as well. Because of the large number of features which must be created in each mold plate, they are difficult (and hence expensive) to manufacture. The large number of features in each mold plate also makes troubleshooting of the system problematic, as it is difficult to isolate a particular feature as the cause of a molding problem. The alignment between the halves of the molds must be precise such that the individual cavities of the mold properly align when the mold is in a closed position, and different rates of thermal expansion across the multicavity mold may cause misalignment of the individual mold cavities, leading to unusable discs. Also, many manufacturers of high density optical discs currently use single cavity molds for the reasons described above. It would be prohibitively expensive to replace those single cavity molds with multicavity molds.
Clearly, it would be desirable to provide an apparatus and method which combines the high quality production of single cavity molds with the high-capacity production of multicavity molds, while not requiring manufacturers to replace their current stock of single cavity molds.
The present invention is a multiple cavity injection molding system which utilizes at least two single cavity injection molds for forming optical discs. Each single cavity injection mold has first and second mating portions which are movable between a closed position in which a mold cavity is formed and an open position in which the disc is removed from the mold cavity. A resin delivery system is operatively coupled to the first mating portion of each of the at least two single cavity injection molds for delivering resin into each of the mold cavities. An ejector system is operatively coupled to the second mating portion of the injection molds for ejecting the molded disc from the mold cavity. The present invention allows existing single cavity molds to be used in a multicavity system.
In a preferred embodiment, the single cavity injection molds are physically and thermally separated from each other, so as to maintain uniform thermal characteristics across each mold cavity. The thermal isolation may be created by providing an air gap between the single cavity molds. In other embodiments, a coolant may be circulated between the single cavity molds to maintain thermal uniformity. The preferred physical and thermal separation of the single cavity molds allows the mating portions of the molds to independently center themselves upon being moved to the closed position, thereby eliminating the effects of thermally induced dimensional changes.